Cutting Tool Coated with a Diamond-Like Carbon Multilayer

ABSTRACT

A coated cutting tool is at least partially coated with a diamond-like carbon (DLC) multilayer of alternating DLC layers and metal nanolayers sandwiched between the DLC layers. The nanolayer comprises a metal or a metal alloy comprising two or more elements selected from of groups IVB, VB, VIB of the periodic table, Al, and Si. Each DLC layer has a thickness of at least 0.2 microns thick and the coating has a thickness of from about 0.5 microns to about 30 microns.

FIELD OF THE INVENTION

The present invention relates to a cutting tools covered at least partially with a multilayer coating comprising layers of diamond-like carbon (DLC) and nano layers of at least one metal selected from groups IVB, VB or VIB of the periodic table, Al and Si.

BACKGROUND OF THE INVENTION

It is known to coat substrates of cutting tools with hard carbon coatings such as diamond-like carbon (DLC) coatings. The crystal structure of DLC coatings is not ordered and it is a mixture of small microcrystallites or nano crystallites of diamond (sp3 bonding) and graphite (sp2 bonding). The ratio of sp3/sp2 in the DLC layer may vary but for use in cutting tools it is greater than about 0.5 and preferably greater than 0.8.

DLC coatings have high corrosion-resistance and are chemically inert at low temperatures, have high mechanical hardness, and a low coefficient of friction. These properties make DLC coatings attractive coatings for cutting tools.

The high internal compressive stress of DLC coatings results in poor adhesion to the substrate. The stress increases with the thickness of the coating and thus, thin layers of DLC are less prone to delamination or to flaking. DLC coatings with an sp3/sp2 ratio of at least 0.75, hardness of at least 4500 Hv that are used for cutting tools are generally limited to a thickness of up to about 2 microns. There is a need for increasing the coating thickness of DLC coatings with high sp3/sp2 ratio while maintaining satisfactory adhesion of the coating to the substrate to improve cutting tool performance and tool life in machining of, for example, non-ferrous materials such as Al—Si alloys, and CFRP (composite fiber reinforced plastics) materials.

One method to improve adhesion is the application of one or more intermediate layers between the DLC coating and the substrate. Known intermediate layers include one or more metal layers such as Ti or Cr, non-metal layers such as Si, ceramic layers such as, for example, carbide or nitride layers such as TiC, TiN or SiC. While metal intermediate layers improve adhesion of the DLC coating to the substrate, this interface is relatively soft which is undesirable for cutting tools.

Another method of improving adhesion and of reducing the internal stress of DLC films are composite coatings of DLC and metal or ceramics such as carbides, nitrides, oxides, borides etc. U.S. Pat. No. 6,143,142 describes a PVD method using targets that contain carbon and one or more components such as Ti, Ni, Cr, Al, Si and W that are in the DLC coating obtained by this method. The internal stress of these composite DLC coatings is less than that of pure DLC coatings. While their adhesion to the substrate is improved and the thickness of these coatings increased, the hardness is likewise less than the hardness of DLC coatings.

A way of increasing the thickness of the DCL coating is a multilayered coating of alternating DLC layers and either a carbide (as disclosed in CN 101081557) or diamond like nanocomposite (DLN) (as disclosed in U.S. Pat. No. 6,228,471). However, the DLN layers are softer than the DLC layers and so the hardness of this multilayered coating is less than a DLC coating.

DLC may be deposited by CVD or by PVD. While DLC coatings that are deposited by PVD are limited in thickness of the coating due to its internal stresses resulting in limited tool life, PVD coatings are suitable for coating sharp cutting edges and for coating complexly shaped cutting edges both of which are of interest for coated cutting tools.

SUMMARY OF THE INVENTION

DLC layers are known to bond well with metal layers and hence can be used as intermediate layers between a DLC layer and a substrate. The present invention is a multilayered coating of layers of DLC with metal nanolayers between the DLC layers. The metal may be one or more metals of groups IVB, VB, and VIB of the periodic table, Al, and/or Si. One advantage of this arrangement is that the thickness of the DLC coating according to the invention may be up to 10 microns, which is thicker than the DLC coatings of the prior art. Another advantage is that the hardness of DLC coating is not reduced by multilayered arrangement since the metal nanolayers are not thick enough to negatively affect the hardness of the DLC coating. The metal nanolayer between the DLC layers may relieve stress while increasing adhesion of the DLC layers.

In accordance with the present invention there is provided a coated cutting tool comprising a substrate and a coating comprising a diamond-like carbon (DLC) multilayer formed over the substrate, the DLC multilayer comprising at least three alternating layers:

at least two DLC layers and at least one nanolayer sandwiched between the DLC layers, wherein:

the DLC layer has a sp3/sp2 ratio from 0.5 to about 0.98 and has a thickness of at least 0.2 microns thick, and

the nanolayer comprising a metal or a metal alloy comprising two or more elements selected from of groups IVB, VB, VIB of the periodic table, Al, and Si.

Typically the nanolayer has a thickness of from about 5 nm to about 200 nm.

In accordance with some embodiments, the coating further comprises a bonding layer between the substrate and the DLC multilayer, the bonding layer comprising at least one layer comprising one or more elements selected from the group consisting of groups IVB, VB, VIB of the periodic table, Al, and Si, or ceramics thereof.

Optionally at least one DLC layer of the DLC multilayer is a composite DLC layer comprising DLC and one or more metals, metal alloys or ceramics.

In one embodiment of the invention all of the DLC layers of the DLC multilayer have the same thickness.

Optionally the thicknesses of the individual DLC layer thicknesses vary throughout the DLC multilayer.

In some embodiments of the invention all of the DLC layers of the DLC multilayer have the same sp3/sp2 ratio.

In accordance with some embodiments, the sp3/sp2 ratios of the individual DLC layers vary throughout the DLC multilayer.

In one embodiment, the DLC layers of the DLC multilayer have a constant sp3/sp2 ratio throughout the thicknesses of the DLC layer.

In accordance with some embodiments, the sp3/sp2 ratio of one or more of the individual DLC layers varies throughout the thickness of the DLC layer.

In accordance with some embodiments, all of the nanolayers of the DLC multilayer have the same thickness.

In accordance with some embodiments, the thicknesses of the individual nanolayers vary throughout the DLC multilayer.

In accordance with some embodiments, the DLC multilayer comprises at least two individual nanolayers and all of the nanolayers comprise the same metals or metal alloys.

In accordance with some embodiments, the DLC multilayer comprises at least two individual nanolayers comprising different metals or metal alloys.

In accordance with some embodiments, the substrate is selected from the group consisting of: high speed steel, hard metals, oxide ceramics, carbide ceramics, boride ceramics, super abrasive materials, PcBN and PCD.

In accordance with some embodiments, the coating has a thickness of from about 0.3 microns to about 30 microns.

BRIEF DESCRIPTION OF THE FIGURES

For a better understanding of the invention and to show how it may be carried into effect, reference will now be made, purely by way of example, to the accompanying drawing.

FIG. 1 is a schematic cross-section of the coating layers of a cutting tool in accordance with one embodiment of the invention.

FIG. 2 shows an exemplary cutting insert having the inventive coating.

With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention; the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a cutting tool at least partially coated with a DLC multilayer that has at least two layers of DLC and a nanolayer sandwiched between the DLC layers. The cutting tool may be a one-piece solid cutting tool, a cutting tool comprising an insert holder and a cutting insert secured therein, an insert holder, or even a cutting insert, by itself.

The DLC layers have a hardness of from about 3000 HV to about 8000 HV (the sp3/sp2 ratio is from about 0.5 to about 0.98). Optionally, the sp3/sp2 ratio may be constant throughout the thickness of individual DLC layers or alternatively may vary by either increasing or by decreasing throughout the DLC layer. All of the individual DLC layers of the DLC multilayer may have the same sp3/sp2 ratio. Alternatively, individual DLC layers of the DLC multilayer have different sp3/sp2 ratios. In some embodiments, the sp3/sp2 ratios may increase or alternatively decrease throughout the DLC multilayer in a direction away from a substrate. In still other embodiments, the sequence of sp3/sp2 ratios of individual DLC layers may be essentially aperiodic throughout the DLC multilayer.

The DLC layers may be from about 0.2 microns to about 1 micron thick. In some embodiments, all the individual DLC layers of the DLC multilayer have the same thickness. In other embodiments, individual DLC layers of the DLC multilayer may vary in thickness. For example, the thickness of individual DLC layers may either increase or decrease in a direction away from a substrate. Alternatively, the sequence of thicknesses of individual DLC layers may be essentially aperiodic throughout the DLC multilayer. One or more of the DLC layers may optionally be doped with metals or ceramics and be a composite layer of DLC and metal or ceramics as known in the art. Individual composite DLC layers of the DLC multilayer may have the same composition or may optionally have varying compositions.

The nanolayer comprises one or more metals selected from groups IVB, VB, VIB of the periodic table, Al, or Si and is from about 5 nm to about 200 nm thick. The nanolayer adheres to the DLC layers but is not thick enough to negatively affect the hardness of the coating. In embodiments having at least two nanolayers, individual nanolayers may optionally be from different metals or metal alloys.

A substrate of the cutting tool may be made a hard metal, high speed steel, oxide, carbide or boride ceramics, or a super abrasive (PcBN or PCD) substrate. A bonding layer is deposited on the substrate and the DLC multilayer is deposited on the bonding layer. The bonding layer may be a monolayer, a multilayer, or one or more ceramics or metals or any of the layers known in the art to improve the adhesion of DCL to the substrate. The top layer of the bonding layer and the at least one nanolayer may optionally be of the same metal or metal alloy.

Example 1

By way of proof of concept, with reference to FIG. 1, the substrate 5 of the cutting tool 10 is of hard metal and the coating is deposited by an arced PVD method. The bonding layer 1 is a thin Ti layer with thickness of 100-200 nm. The DLC multilayer 4 is about 3.9 microns thick and made of 12 DLC layers 2 of 0.3 microns alternating with nanolayers 3 of Ti of 30 nm. The coating may be formed on one or more surfaces of a cutting insert 20, which is the cutting tool shown in FIG. 2. In some embodiments, a cutting tool may only be partially coated, such as in regions proximate its cutting edges 22, while the remainder of the cutting tool is left uncoated with the inventive coating. 

1. A coated cutting tool comprising a substrate and a coating comprising a diamond-like carbon (DLC) multilayer formed over the substrate, the DLC multilayer comprising at least three alternating layers: at least two DLC layers and at least one nanolayer sandwiched between the DLC layers, wherein: the DLC layer has a sp3/sp2 ratio from 0.5 to about 0.98 and has a thickness of at least 0.2 microns; and the nanolayer comprises a metal or a metal alloy comprising two or more elements selected from of groups IVB, VB, VIB of the periodic table, Al, and Si.
 2. The coated cutting tool according to claim 1, wherein the nanolayer has a thickness of from about 5 nm to about 200 nm.
 3. The coated cutting tool according to claim 1, wherein the coating further comprises a bonding layer between the substrate and the DLC multilayer the bonding layer comprising at least one layer comprising one or more elements selected from the group consisting of groups IVB, VB, VIB of the periodic table, Al, and Si, or ceramics thereof.
 4. The coated cutting tool according to claim 1, wherein at least one DLC layer of the DLC multilayer is a composite DLC layer comprising DLC and one or more metals, metal alloys or ceramics.
 5. The coated cutting tool according to claim 1, wherein all of the DLC layers of the DLC multilayer have the same thickness.
 6. The coated cutting tool according to claim 1, wherein the thicknesses of the individual DLC layer thicknesses vary throughout the DLC multilayer.
 7. The coated cutting tool according to claim 1, wherein all of the DLC layers of the DLC multilayer have the same sp3/sp2 ratio.
 8. The coated cutting tool according to claim 1, wherein the sp3/sp2 ratios of the individual DLC layers vary throughout the DLC multilayer.
 9. The coated cutting tool according to claim 1, wherein the DLC layers of the DLC multilayer have a constant sp3/sp2 ratio throughout the thicknesses of the DLC layer.
 10. The coated cutting tool according to claim 1, wherein the sp3/sp2 ratio of one or more of the individual DLC layers varies throughout the thickness of the DLC layer.
 11. The coated cutting tool according to claim 1, wherein all of the nanolayers of the DLC multilayer have the same thickness.
 12. The coated cutting tool according to claim 1, wherein the thicknesses of the individual nanolayers vary throughout the DLC multilayer.
 13. The coated cutting tool according to claim 1, wherein the DLC multilayer comprises at least two individual nanolayers and all of the nanolayers comprises the same metals or metal alloys.
 14. The coated cutting tool according to claim 1, wherein the DLC multilayer comprises at least two individual nanolayers, each comprising different metals or metal alloys.
 15. The coated cutting tool according to claim 1, wherein the substrate is selected from the group consisting of: high speed steel, hard metals, oxide ceramics, carbide ceramics, boride ceramics, super abrasive materials, PcBN and PCD.
 16. The coated cutting tool according to claim 1, wherein the coating has a thickness of from about 0.3 microns to about 30 microns.
 17. The coated cutting tool according to claim 1, wherein the multilayer further comprises at least one ceramic layer between the at least two DLC layers.
 18. A coated cutting insert comprising a substrate and a coating comprising a diamond-like carbon (DLC) multilayer formed over the substrate, the DLC multilayer comprising at least three alternating layers: at least two DLC layers and at least one nanolayer sandwiched between the DLC layers, wherein: the DLC layer has a sp3/sp2 ratio from 0.5 to about 0.98 and has a thickness of at least 0.2 microns; and the nanolayer comprises a metal or a metal alloy comprising two or more elements selected from of groups IVB, VB, VIB of the periodic table, Al, and Si. 